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Another option to create the pittsburgh seam, is to find a local sheet metal or industrial insulation shop. They will have a machine the puts the seam on your metal for you, short of using a sheet metal brake as described. It is called a lock former. Then, the mating piece is made by running the cap through an easy edger.
If you have your metal cut, you need to add 7/8" to the side panels to allow for the lock former, as it will shrink this much due to the folding process. So that is 1 3/4" total to your piece. For the cap, add an extra 3/16" typically, that is the bend that will lock into the sidewall, then use a sheet metal hammer to finish off the edge, then solder.
I have this equipment, as by trade I am an industrial insulator and sheet metal worker. I have been thinking of making up some tanks. If you need, I may be able to help fabricate it.